1. Field of the Invention
This invention relates to a control device for controlling a driving current of a permanent magnet motor (PM motor) having a pole saliency.
2. Description of Related Art
FIG. 5 shows a schematic structure of a salient pole type permanent magnetic motor (PM motor), where an end surface of the motor is illustrated. A predetermined number of permanent magnets 12 and salient poles 14 having a specified interval between each other are disposed on an outer circumference surface of a rotor yoke 10 opposing a stator 16. When a torque current Iq flows into a stator winding (not shown), a magnet torque is generated by the current Iq and flux .phi. of the permanent magnets 12, and thus a rotor rotates. In addition, when an excitation current Id flows into the stator winding, a reluctance torque arisen due to the presence of the salient poles 14 formed of magnetic material. Therefore, a torque T of the motor of this kind is generally equal to a value which is expressed by the following equation. A first term within parenthesis on right-hand side of the equation represents a magnet torque, and a second term of the same represents a reluctance torque. EQU T=p(.phi.+.DELTA.L.multidot.Id)Iq
where "p" represents a number of pole pairs, .phi. [wb] a flux of the magnets, .DELTA.L=Ld-Lq, Ld[H] a d-axis component of a primary inductance of the motor (d-axis inductance), Lq[H] a q-axis component of a primary inductance of the motor (q-axis inductance), Id[A]=I.multidot.sin .theta. a d-axis driving current (excitation current), Iq[A]=I.multidot.cos .theta. a q-axis driving current (torque current), I[A] an amplitude of motor current, and .theta. [deg] a phase of motor current.
For controllably driving the motor having such a construction, a current vector (I,.theta.) of the motor is controlled depending on a required torque, i.e., reference torque. On the other hand, for the PM motor, a counterelectromotive force is generated following increase of a number of rotations N, and in turn, an angular frequency .omega. of the motor current. Thus, the control proceeds in a lower rotational speed region to make the excitation current Id 0 (non-salient pole machine) or a predetermined value (salient pole machine), and in a higher rotational speed region to give a negative excitation current Id so as to cancel the counter electromotive force. As a result of such control, an upper limit of an output torque T of the motor is limited by a specified value (maximum torque) which is equal to or less than a rotational speed where a counterelectromotive force begins to exceed a power supply voltage, and an upper limit of an output power=torque T.times.rotational speed N of the motor is limited by a specified value (maximum power) which is more than such rotational speed (see FIG. 6). A reference torque, which is determined depending on the characteristic as described above, is converted into an amplitude I and a phase .theta. of the motor current, namely, a reference current (I, .theta.) (see FIG. 7). The reference current obtained by conversion is used for controlling switching elements constituting an electric power circuit (for example, an inverter) which controls current supply to the motor.
The PM motor having the pole saliency produces a reluctance torque in addition to a magnetic torque as described above. Therefore, a torque of the motor can always be controlled for the maximum value by controlling a phase .theta. of the motor current so that a sum of the magnet torque and the reluctance torque comes maximum, as described in Hatanaka et al., "Maximum Torque Control of Inverse-Salient Pole Type PM Motor" 1991 NATIONAL CONVENTION RECORD I.E.E. Japan, April, No. 580, pp.6-10.
However, the permanent magnet has a property of generating demagnetization (this is also referred to as "demagnetism" or "degaussing"), where a magnetic flux .phi. is reduced with use while depending on temperature, environment, or the like. With demagnetization occurring, the magnet torque gradually becomes not generated, and in addition, a sudden increase of temporary thermal load and damage of the permanent magnet arises to produce the same phenomenon. For these reason, when the magnet torque is being decreased, the motor torque T that is a sum of the magnet torque and the reluctance torque is also decreased as shown in FIG. 8, and it becomes difficult to realize a torque (reference torque) as required (in the drawing, .phi. represents an electromotive force, and is expressed as a percentage of a normal value).
A target value of the phase .theta. of the motor current, i.e., a target value of the phase .theta. set such that a sum of the magnet torque and the reluctance torque becomes maximum, is normally set under the precondition that demagnetization does not arise on the permanent magnet. Actually the phase .theta., where a sum of the magnet torque and the reluctance torque becomes maximum, is varied when demagnetization arises. Hence, in a state where the demagnetization arises, only a small torque is obtained even in comparison with a maximum torque which can be outputted in the demagnetization state.